Re: [PATCH v2 2/3] perf x86: Add compaction function for uncore attributes

["Sudarikov, Roman" <roman.sudarikov@linux.intel.com>]

On 10.12.2019 13:37, Peter Zijlstra wrote:
> On Tue, Dec 10, 2019 at 12:14:50PM +0300, roman.sudarikov@linux.intel.com wrote:
>> From: Roman Sudarikov <roman.sudarikov@linux.intel.com>
>>
>> In current design, there is an implicit assumption that array of pointers
>> to uncore type attributes is NULL terminated. However, not all attributes
>> are mandatory for each Uncore unit type, e.g. "events" is required for
>> IMC but doesn't exist for CHA. That approach correctly supports only one
>> optional attribute which also must be the last in the row.
>> The patch removes limitation by safely removing embedded NULL elements.
>>
>> Co-developed-by: Alexander Antonov <alexander.antonov@intel.com>
>> Signed-off-by: Alexander Antonov <alexander.antonov@intel.com>
>> Signed-off-by: Roman Sudarikov <roman.sudarikov@linux.intel.com>
>> ---
>>   arch/x86/events/intel/uncore.c | 22 ++++++++++++++++++++++
>>   1 file changed, 22 insertions(+)
>>
>> diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
>> index 24e120289018..a05352c4fc01 100644
>> --- a/arch/x86/events/intel/uncore.c
>> +++ b/arch/x86/events/intel/uncore.c
>> @@ -923,6 +923,22 @@ static void uncore_types_exit(struct intel_uncore_type **types)
>>   		uncore_type_exit(*types);
>>   }
>>   
>> +static void uncore_type_attrs_compaction(struct intel_uncore_type *type)
>> +{
>> +	int i, j;
>> +	int size = ARRAY_SIZE(type->attr_groups);
>> +
>> +	for (i = 0, j = 0; i < size; i++) {
>> +		if (!type->attr_groups[i])
>> +			continue;
>> +		if (i > j) {
>> +			type->attr_groups[j] = type->attr_groups[i];
>> +			type->attr_groups[i] = NULL;
>> +		}
>> +		j++;
>> +	}
>> +}
> GregKH had objections to us playing silly games like that and made us
> use is_visible() for the regular PMU driver. Also see commit:
>
>    baa0c83363c7 ("perf/x86: Use the new pmu::update_attrs attribute group")


Removed theuncore_type_attrs_compaction() function and implemented Kan's
suggestion to replace NULL events_group by the empty attributes group:


diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
index e8532923bd45..110b3603f56f 100644
--- a/arch/x86/events/intel/uncore.c
+++ b/arch/x86/events/intel/uncore.c
@@ -923,6 +923,14 @@ static void uncore_types_exit(struct 
intel_uncore_type **types)
                 uncore_type_exit(*types);
  }

+static struct attribute *empty_attrs[] = {
+       NULL,
+};
+
+static const struct attribute_group empty_group = {
+       .attrs = empty_attrs,
+};
+
  static int __init uncore_type_init(struct intel_uncore_type *type, 
bool setid)
  {
         struct intel_uncore_pmu *pmus;
@@ -968,7 +976,8 @@ static int __init uncore_type_init(struct 
intel_uncore_type *type, bool setid)
                         attr_group->attrs[j] = 
&type->event_descs[j].attr.attr;

                 type->events_group = &attr_group->group;
-       }
+       } else
+               type->events_group = &empty_group;

         type->pmu_group = &uncore_pmu_attr_group;